Quoted from terryb:
You have several not used switches in column 7 showing as closed and probably others, but I didn't check them all. So you have several issues going on. I doubt if the ribbon cable being off by one pin would have damaged the switch matrix, though it could have damaged other circuits.
Take all of the balls out of the game and check which switches are showing closed. If any are closed that shouldn't be note them down (please use switch numbers rather than descriptions). Then go through and check all switches and note any that don't work.
Also please provide hi-res, in-focus photos of both sides of U20.
I would not suspect the LM339's at this point unless you're having row issues that you did not mention.
Unfortunately I'm not having a beer.
TerryB I documented all the closed switches they are 24, 51, 52, 53, 54, 55, 56, 57, 61 ,62, 63, 64, 65 , 66, 67, 71, 72, 73, 74, 75 , 76, 77, 81, 82, and 87. The switches that dont work are all on column 6 I think some of the closed switches are opto's so they should be closed but 61, 62 , 63, 64, 65, 66, 67, 68 are all on column 6. A little history this game worked fine for a game or two I turned it off came back to it about 15 min later and was getting the column 6 switch errors that is when I reseated the cables and then started getting the other errors after i put the driver/mpu ribbon cable on wrong. All the chips on the mpu were soketed prior to me owning it. This game has been rock solid for almost 3 years. I have replaced the U20 now 2 times once when i researched the switch errors and after I noticed the ribbon cable being off. The ribbon cable was all the way off on a full row see pic above I have a feeling this might have caused the magnet errors i am getting along with the column 6 errors because I did'nt notice those before i messed up putting the cable on.